The present invention relates to a data transfer system which is designed to perform data transfer, for example, between CPUs or between a CPU and one or more terminal devices with a smaller number of control lines.
An asynchronous data transfer system of the conventional handshake system is disclosed in, for instance, U.S. Pat. No. 3,810,103 "Data Transfer Control Apparatus" issued on May 7, 1974. In this prior art, one data source which is a control side and a plurality of data receiving sides are interconnected via a data line for transferring data from the former to the latter and three control lines for controlling the data transfer. The three control lines are respectively an RFD line for sending to the control side a signal indicating that each receiving side is ready for receiving data (Ready for Data signal), a DAV line for sending to the receiving sides a signal indicating that data set by the control side on the data line is valid (Data Valid signal), and a DAC line for sending to the control side a signal indicating that each receiving side has received the DAV signal and accepted the data on the data line. Since such transfer control from the establishment of what is known as a HANDSHAKE between the data source and the receiving sides to the completion of one data transfer is carried out by using such three independent control lines, the efficiency of utilization of the control lines is poor and the cost of a transmission cable including the data line, too, inevitably becomes higher by that.